Switched power converters have largely replaced linear regulators due to their higher efficiencies and smaller size and weight for a given power-handling capability. Switched converters have themselves evolved from lower-frequency duty cycle controlled pulse-width modulated designs to current frequency-controlled designs.
FIG. 1 is a prior-art schematic diagram of a frequency-controlled voltage converter 100. For purposes of this background discussion, the voltage converter 100 is representative of frequency-controlled power converters generally, including both voltage and current converters. A pulse-width modulator (“PWM”) 105 generates a rectangular control signal 108, typically a square wave although not necessarily so. Edges of the control signal 108 control the switching of transistors in a converter power stage 115. The on and off states of the power stage switching transistors alternately allow and prevent current flow to and from various energy storage devices within the power stage 115. For a frequency-controlled voltage converter such as the converter 100 of FIG. 1, the frequency of the control signal 108 determines the power stage switching frequency and consequently the converter output voltage V_OUT 120. Although the example power stage 115 of FIG. 1 happens to implement an LLC resonant type converter, frequency-controlled power stages may be of various designs.
The level of V_OUT 120 is maintained for various output load conditions via a negative feedback control system. A sample of V_OUT 120 is fed back to the PWM 105 to increase or decrease the frequency of the control signal 108 to compensate for decreases in V_OUT 120 due to increased output loading or increased converter input voltage, etc. The feedback control system may be analog or digital. In the case of a digital control system, an analog-to-digital converter (“ADC”) samples V_OUT 120 and sends a digital value corresponding to the instantaneous magnitude of V_OUT 120 to an inverting input 135 of an output control circuit 125. The V_OUT 120 sample value is compared by the control circuit 125 to a reference value sent to the control circuit 125 on an input 130. Alternatively, or in addition, the reference value may be stored in a register of the control circuit 125. The control circuit 125 compares the V_OUT 120 sample value to the reference value and may perform various control loop compensation operations. The control circuit 125 sends a frequency-controlling signal or value to the PWM 105 on an input 140. In the case of a digital PWM input, the frequency-controlling value may represent either a frequency or a period of the PWM output signal. The PWM 105 generates the switching control signal 108 at a frequency corresponding to the frequency-controlling value or signal received from the control circuit 125.
The control circuit 125 may perform control loop compensation operations as previously mentioned. Such compensation operations are designed to improve the function of maintaining a constant V_OUT 120 in light of changing converter voltage input, output loading, noise on the input voltage waveform or noise seen at the converter output, etc. However, such loop compensation operations generally utilize characterization information for the converter power stage, including the power stage frequency response or transfer function. The latter are dependent upon the design and component values of each individual converter's power stage. Even for a known design, the transfer function of a power stage associated with a frequency-controlled power converter is difficult to determine due to the complex behavior and interactions of the various energy storage components.
FIG. 2 is a prior-art block diagram of a frequency-controlled voltage converter 200 such as the converter 100 previously described with reference to FIG. 1. The converter 200 includes the PWM 105, the output control circuit 125 and the ADC 133, all as previously described for the converter 100 of FIG. 1. The converter 200 also includes the power stage 215, and is included merely to illustrate that the example power stage 215 referred to subsequently herein may be of any frequency-controlled design.